I have two columns in my db: start_date and end_date, which are both DATE types. My code is updating the dates as follows:
start_date
end_date
DATE
You can use strtotime() as in Gazler's example, which is great for this case.
If you need more complicated control use mktime().
$end_date = mktime(date("H"), date("i"), date("s"), date("n") + 1, date("j"), date("Y"));